Ship rotation in GUI needs to be interpolated to prevent "snapping".

Currently, the ship rotation is calculated based on the current heading of the ship. This can change dramatically between updates and causes the ships to "snap" after the last server update. Interpolating to the new rotation over a few frames should make the ship movement look smoother.

The rate of interpolation should be absolute (rotating 15 degress should take the same time as rotating 180) to make sure the ships don't look like they are "sliding" through space.